The presence of several sulfydryl proteinases have been reported in various tissues; however, only one of them has been purified and studied extensively. These enzymes are probably involved in the intracellular protein turnover. Tissue sulfydryl proteinases will be separated from other tissue proteinases by chromatography on mercurial-Sepharose. Subsequently, individual sulfydryl proteinases will be purified by affinity chromatography. These enzymes will be characterized with respect to their molecular properties; their active sites will be investigated by means of active-site-directed reagents and bifunctional inhibitors; their mechanism of action and substrate specificity will be investigated; and amino acid sequence determined. These studies may eventually help in elucidating the role of the sulfydryl proteinases in protein turnover, and in pathological conditions such as muscular dystrophy and some forms of arthritis.